Coating of shaped articles



United States Patent Uhlice 3,022,198 Patented Feb. 20, 1962 3,022,198COATING F SHAPED ARTICLES Harold G. Hahn, Midland, Mich., assignor toThe Dow Chemical Company, Midland, Mich., a corporation of DelawareFiled July 5, 1960, Ser. No. 40,548 5 Claims. (Cl. 117-138.8)

This invention relates to a method of treating the surfaces of shapedarticles such as lms, sheets, panels, and the like fabricated fromalkenyl aromatic resins with a lacquer prepared from copolymers ofvinylidene chloride and acrylonitrile.

The invention is particularly adapted to be practiced with swellable,integral, solid styrene polymer film or sheets. Such a polymer, whichmay have an essentially linear molecular configuration or be in across-linked form, is employed herein as being generally representativeof alkenyl aromatic compounds of the type that contain at least about 50weight percent of at least one polymerized alkenyl aromatic compound ormonomer having the general formula: Ar-CR=CH2, wherein R is hydrogen ormethyl and Ar is an aromatic radical, advantageously of the benzeneseries, of from 6 to l0 carbon atoms (including the carbon atoms in anyring substituents on the aromatic nucleus). Thermoplastic polymers andcopolymers of styrene and polymers and copolymers of alphamethylstyrene, ar-methyl styrene (or vinyl toluene), the several monoanddi-chlorostyrenes and ar-dimethyl styrenes, including copolymers thereofwith such materials as vinylidene chloride and acrylonitrile;cross-linked polyfunctional substances as divinyl benzene; and graftcopolymers with other polymeric substances (such as other elastomericpolymers) may frequently be utilized with benets commensurate with or inexcess of those which are derivable from employment of polystyrenealone.

Vinylidene chloride-acrylonitrile copolymer coatings are particularlydesirable and advantageous because of their high chemical resistance andexceptionally low permeability to various gases and vapors.

In the past, attempts to apply acrylonitrile-vinylidene chloridecopolymers by means of lacquer systems to shaped articles fabricatedfrom alkenyl aromatic resins has been notably unsuccessful. There hasbeen no solvent known to dissolve a vinylidene chloride-acrylonitrilecopolymer which was not also a strong solvent for an alkenyl aromaticresin. Subsequently, attempts to apply vinylidene chloride-acrylonitrilccopolymers from such a lacquer system resulted in a severe solventattack on the alkenyl aromatic resin substrate frequently causingcrazing, warping, solution, or general destruction of the desirablesurface properties of the substrate.

Benets and advantages of a vinylidene chloride-acrylonitrile copolymercoating on an alkenyl aromatic resin substrate could only be obtained ifsome suitable means were employed to protect the substrate from eitherlacquer solvent during its application or by applying the vinylidenechloride-acrylonitrile copolymer from a latex system. Both techniquesare somewhat disadvantageous as, in the case of a latex coating,adhesion tothe substrate is diflicult -to obtain and the coating iscontaminated by wetting agents and other additaments which are necessaryto prepare the aqueous dispersion of the coating polymer. An aqueousdispersion usually does not have long term shelf stability and issubject to a gradual 4change in characteristics due to decomposition andslow agglomeration of the dispersed particles. If a protective base coatis applied to an alkenyl aromatic substate prior to the `application ofa lacquer system, a double coating operation is required which iseconomically disadvantageous.

it is an object of ythis invention to provide a lacquer compositionsuitable for the coating of shaped objects prepared from alkenylaromatic resins.

It is a further object of this invention to provide a method of coatingshaped objects prepared from an alkenyl aromatic resin.

It is a further object of this invention to provide a method of coatinga shaped article prepared from an alkenyl `aromatic resin with a coatingcomprising a vinylidene chloride-acrylonitrile copolymer.

These benefits and other advantages may be obtained by treating thesurface of a shaped article prepared from an alkenyl `aromatic resinwith a solution comprising a copolymer of from about 15 to 25 percent byweight acrylonitrile and to 75 percent by weight vinylidene chloride ina volatile solvent comprising ethyl glycolate and subsequently removingsaid volatile solvent therefrom.

The drawing shows a ilow sheet representing the steps of the invention.y

The vinylidene chloride-acrylonitrile copolymer employed in the practiceof the invention may be prepared by any of the conventional andWell-known methods of polymerization such as emulsion, suspension,solution and mass or bulk polymerization.

The ethyl glycolate employed in the practice of the inventionadvantageously may be ladmixed with a minor portion (i.e., up to about35 percent by Weight) of another solvent such Vas acetone, methyl ethylketone, dimethyl formamide, tetrahydrofuran, ethyl acetate and the like.In cases where such a diluent or secondary solvent is employed whichcould attack the alkenyl aromatic resin alone, its vapor pressure shouldbe equal to or greater Ithan the vapor pressure of ethyl glycolate atthe same temperature. If the vapor pressure of the secondary or.

diluent solvent is less than that of ethyl glycolate, the concentrationof the secondary solvent in the solvent mixture often will increase asevaporation takes place and frequently attack the alkenyl aromaticsubstrate.

Usually it is beneficial to maintain at least about 65 percent by weightof the total solvent employed in the lacquer as ethyl glycolate, and,frequently, it is advantageous to employ about percent by weight of thetotal solvent as ethyl glycolate.

The use of minor portions of solvents other than in admixture with ethyllglycolate frequently will improve the adhesion of the vinylidenechloride-acrylonitrile copolymer to the alkenyl aromatic substrate.Usually, for conventional purposes the adhesion of Ithe coating isadequate without the dilution of the lacquer system With a mutualsolvent for both polymers. When extremely thin films or sheets ofpolystyrene or like alkenyl aromatic resinous yarticles are beingcoated, it is :advantageous to maintain a relatively high portion ofethyl glycolate in the lacquer solvent in order to avoid solvent attackand deformation of the thin substrate.

The lacquer mixture may readily be prepared by dissolving solidvinylidene chloride-acrylonitrile copolymers and ethyl glycolate by anyconventional methods such as agitating a suspension of the polymer andsolvent at room temperature, by heating in combination with agitationand the like. Advantageously, in cases where Itime is not a signicantfactor, the solvent and copolymer may be placed in the same vessel andallowed to stand until complete solu-tion takes place. The concentrationof copolymer in solvent will vary with the particular coating methodemployed and characteristics of the copolymer used. lUsually thecopolymer will be about l0 to 2O percent by weight of the total lacquermixture.

Lacquers comprising ethyl glycolate and acrylonitrilevinylidene chloridecopolymers are readily applied to shaped articles prepared from alkenylaromatic resins by conventional means, such as spraying, brushing,dipping,

rolling, and the like. Generally, it is advantageous to apply a lacquerat a temperature offrom about 20-40 C.V when employing manualtechniques. By using automatic coating equipment temperaturesv upto andexceeding'V the heat distortion point of vthe alkenyl aromatic resin ifmay be employed, that is, up to about 100 C;

By way of further illustration, a copolymer of acrylonittileV `andvinylidene chloride was prepared by copolymerizingv 20 pants ofacrylonitrile and 80 parts of vinylidene chloride in an aqueous solutionof sodium dihexyl sulphosuccinate. The aqueous polymer-ization mediumemployed containing as polymerization catalysts about 0.5 Y

percent of potassium persulphate based on monomer and about 2.5 percentsodium dihexyl sulphosuccinate based.

on monomer was placed in a closed reactor and heated to a temperature ofabout 45 C. A monomer mixture consisting of 20Yparts by weight ofacrylonitrile and 80 parts Y by weight vinylidene chloride was added tothe aqueous hours. A clear transparent lacquer resulted. Y Theacrylonitrile-viny-lidene chloride copolymer-ethyl glycolate lacquerwas' spread on a sheet of polystyrene about l mil in Y thickness. Thelacquer coated polystyrene substrate was Y dried at a temperature of 60C. for a period of 45 minutes. 'Ihe resultant coated sheet orV film wasclear and showed no evidence of crazing or distortion.

In a similar manner other alkenyl aromatic resins may be coated withvinylidene chloride-acrylonitrile copolymers preparedV #from monomermixtures containing from 15 to 25 percent by weightacrylonitrile and 85to 75 percent by weight vinylidene chloride.

As is apparent, the methodris susceptible of being embodied with variousalterations and modications from that which is being described in thepreceding description and specification. For this reason, it is to beunderstood that all of the foregoing -is merely intended to beillustrative and is not-to be construed or interpreted as beingrestrictive or otherwise limitative of the present invention exceptV asset forth in the appended claims. f

What is claimed is: f

. l. A method of coating a shaped article comprising treating thesurface of a shaped article prepared from an alkenyl momatic resinhaving chemically combined in its polymer molecule at least about 50weight percent of at least one polymerized alkenyl aromatic compoundhaving -the general formula:

wherein R is selected from the group consisting of hydrogen and methyland Ar is Ian aromatic group of the benzene series and the group-CR=C-H2 is attached directly to the aromatic ring with a solutioncomprising a copolymer of from about l5 to 25 percent by weightacrylonitrile and 85 to 75 percent by weight vinylidene chloride in avolatile solvent comprising'ethyl glycolate and subsequently removingthe volatile solvent therefrom. Y 2. The method of claim 1, wherein saidalkenyl aromatic resin is polystyrene.

3. The method of claim Y1, wherein said shaped article is `a iilm about1 mil in thickness.

4. 'Ihe method of claim l, wherein said solvent consists `of ethylglycolate.

y 5. The. method of claim l', wherein saidsolvent is about V65percent-by `weight ethyl glycolate.

No references cited.

1. A METHOD OF COATING A SHAPED ARTICLE COMPRISING TREATING THE SURFACEOF A SHAPED ARTICLE PREPARED FROM AN ALKENYL AROMATIC RESIN HAVINGCHEMICALLY COMBINED IN ITS POLYMER MOLECULE AT LEAST ABOUT 50 WEIGHTPERCENT OF AT LEAST ONE POLYMERIZED ALKENYL AROMATIC COMPOUND HAVING THEGENERAL FORMULA: